Molten carbonate fuel cell systems can be used in the production of electricity. These systems typically comprise a reformer for converting hydrocarbon fuels to hydrogen and byproducts, a burner, and a plurality of molten carbonate fuel cells. The fuel cells operate such that oxygen contained in an oxidant stream reacts with carbon dioxide and free electrons at a cathode to produce carbonate ions. These carbonate ions migrate across a molten carbonate electrolyte to an anode where they react with hydrogen contained in a fuel stream to produce water, carbon dioxide, and free electrons. The free electrons pass through an external load back to the cathode, thereby producing electricity, while the carbon dioxide, water, and any remaining hydrogen exit the anode in the fuel stream.
In molten carbonate fuel cell systems, the fuel and oxidant streams are often contaminated or can become contaminated with sulfur compounds such as sulfur dioxide, sulfur trioxide, and hydrogen sulfide. These sulfur compounds can poison various components of the fuel cell system including the anode and the catalyst used in the reformer. The anode is readily poisoned by contact with sulfur compounds in amounts exceeding about 1 to 2 parts per million (ppm) by volume while the reformer catalyst is poisoned at very low sulfur concentrations, even below about 0.1 ppm by volume. Poisoning the anode reduces its activity and therefore its ability to convert hydrogen and carbonate ions to water, carbon dioxide, and free electrons while poisoning the reformer catalyst reduces its activity and therefore its ability to convert hydrocarbon fuels to hydrogen. As a result of this poisoning, the activity of the anode and the reformer catalyst, and the life of the molten carbonate fuel cell system are all reduced.
Reformer catalyst poisoning has conventionally been eliminated by purifying the fuel stream prior to its introduction to the reformer. However, the fuel stream is not the only source of sulfur. In a molten carbonate fuel cell sulfur is also introduced by the oxidant stream. This sulfur can concentrate within the molten carbonate fuel cell and poison the anode or reformer catalyst within the molten carbonate fuel cell system. In the molten carbonate fuel cell system, the oxidizing conditions at the cathode cause the molten carbonate electrolyte to have a high affinity for sulfur compounds. As a result, the amount of sulfur trapped within the molten carbonate fuel cell system increases with time.
Sulfur is typically introduced to the molten carbonate fuel cell system in the oxidant stream which is directed to the cathode where it is converted to sulfate ions. The sulfate ions migrate across the molten carbonate electrolyte to the anode where they are converted with hydrogen to hydrogen sulfide and released into the fuel stream. The fuel stream then exits the anode and is directed to a burner where it is burned. Within the burner, the hydrogen sulfide is converted to sulfur dioxide and sulfur trioxide. The burned stream is then directed along with the oxidant stream back to the cathode. Although a portion of the stream exiting the cathode is generally vented, the high affinity and capture of sulfur at the cathode results in an essentially sulfur-free cathode exhaust stream. Therefore, none of the sulfur is vented. The sulfur concentration simply continues to build up within the molten carbonate fuel cell system, thereby compounding the sulfur poisoning problem.
What is needed in the art is a means for removing sulfur compounds from a molten carbonate fuel cell system to prevent contamination of the reformer catalyst and the anode.